Speedometer Problem
New User
Joined: Nov 2000
Posts: 24
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Speedometer Problem
The speedometer in my 1987 E-350 occasionally adds about 7-8 miles per hour to the actual speed. It seems to happen while driving 65mph or higher with the cruise control on. It will smoothly climb up and stay at the "new" speed for a while then drop back down. It seems that the lower speed is the true speed but its hard to tell with a stopwatch and milemarkers while driving. Even though there is an electronic speed sensor, I think that is only for the cruise and the speed comes from a cable. I've tinkered with cars all of my life and this gets me to thinking that I'm not even sure how a speedometer works. I've had cables that need lubed cause a jittery needle, or a worn gear cause no reading, but this is a new one to me. Any ideas?
ken
ken
Postmaster
Joined: Mar 2001
Posts: 4,968
Likes: 3
From: So Cal
Speedometer Problem
Not sure if this is your problem and not sure if it applies to your year model.
On my 94 F250 I had the identical problem, which gradually got worse over time. Over time it also caused the ABS light to come on(I doubt an 85 would have ABS but who knows?). Anyway, at least since the 90s, most Ford trucks appear to have an electronic speed sensor installed on the differential. It is about the size of a walnut and at least on my truck it is located high on the front driver side of the differential. It has a few wires coming out of it which should make it look odd to mechanics who are used to the old mechanical speedometers. It is the only thing on the diff with wires as far as I can tell on my truck. It is easy to replace and it costs I think $12 if you do it yourself. The dealer will charge $60, what a rip off, you don't even have to put the truck on a lift to do this job.
Hope this helps.
Jim Henderson
On my 94 F250 I had the identical problem, which gradually got worse over time. Over time it also caused the ABS light to come on(I doubt an 85 would have ABS but who knows?). Anyway, at least since the 90s, most Ford trucks appear to have an electronic speed sensor installed on the differential. It is about the size of a walnut and at least on my truck it is located high on the front driver side of the differential. It has a few wires coming out of it which should make it look odd to mechanics who are used to the old mechanical speedometers. It is the only thing on the diff with wires as far as I can tell on my truck. It is easy to replace and it costs I think $12 if you do it yourself. The dealer will charge $60, what a rip off, you don't even have to put the truck on a lift to do this job.
Hope this helps.
Jim Henderson
